But Number 10 later claimed that the prime minister's remarks were "over-interpreted". In fact, Cameron was apparently referring to ministers' duty to maintain collective responsibility as the negotiations between Westminster and Brussels continue.

The prime minister later made his own statement on the issue, claiming that he was "misinterpreted". The saga adds to the tension around the EU referendum within the party as a number of Eurosceptic Tories have set up their own pressure group, Conservatives for Britain.

The former care minister warned that the government's crackdown on legal highs would be ineffective and create a new criminal class. Lamb, who is up against Tim Farron in the race to succeed Nick Clegg, also admitted that his party faces an existential crisis.

"If you think about people in society, millions share many of our values but don't associate themselves with the Liberal Democrats. Our task is to convince those people that we are the party that represents their values," he said.

The former foreign secretary told CNN that his brother Ed and colleagues took the reds backwards, instead of building on the New Labour years. He also told The Times that the result was "doubly painful" because of his brother's involvement.

"There is no consolation in any sense of vindication, frankly, because I care about the country and I care about the party," he said.

David will make his return to British politics when he makes a keynote speech at the Institute of Directors annual convention in London on 12 September, less than a month after Ed's successor is announced.